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Chapter 1 -- Introduction (continued)

What is Ecosystem Integrity?

Defining ecosystem integrity for rivers and streams

More final revisions coming 2/1/06.

An understanding of "ecosystem integrity" is essential for evaluating stream habitats and for prioritizing restoration activities. Let's begin by defining the terms "ecosystem" and "ecosystem integrity". An ecosystem is comprised of all the organisms in an area interacting with their environment (Odum 1983). Ecosystem integrity, also known as ecosystem health, is "a desired condition of ecosystems in which productivity of resources and ecological values, including diversity, are resilient to disturbance and sustainable for the long term" (Reynolds 1995). In other words, ecosystem integrity involves maintaining biodiversity, biological productivity, and ecosystem processes. Important aspects of ecosystem integrity include energy flow through the food web, water and nutrient cycles, disturbance/recovery cycles, biotic diversity, evolutionary processes, and human influences. These components and processes function at various rates and across multiple scales. Maintaining ecosystems requires maintaining the processes. It is very difficult or impossible to sustain individual pieces of an ecosystem unless ecosystem processes are also sustained.

The term "ecosystem health" is misleading and is not recommended. The word "health" implies that the system will always try to return itself to a specific equilibrium, such as at a certain body temperature and heart rate. In contrast, ecosystems are constantly evolving and do not maintain themselves in equilibrium.

To evaluate ecosystem integrity, many factors must be considered in the context of the entire system. A vision of the system as a whole is needed in order to recognize connections, patterns, relative importance, and root causes (Reid et al. 1994, Orr 1995). Ecosystem components and processes interact across virtually every spatial and temporal scale (Table 3). In the case of stream habitats, there are three categories of processes and elements that most influence ecosystem processes and integrity: 1) soils/geomorphology; 2) hydrology; and 3) biota (Kauffman et al. 1997).


Table 3. Examples of spatial scales (Cooperrider and Garrett 1995).

Geographic scale:

Species related to this scale:

Processes at this scale:


Anadromous fisheries, neotropical migrant birds

Climatic change


Deer, elk, river otter, bear

Fire and flood regimes


Small mammals, amphibians, reptiles, plant species



Ecosystems elements and processes change over time and it is important to evaluate the patterns of change and the causes for changes. Contrary to the mythical "balance of nature", ecosystems are always changing and do not return to a particular equilibrium condition after disturbance (Botkin 1990, 1994). Ecosystem structure and composition fluctuate and evolve in response to many controlling factors. Over the long term, controlling influences include climatic change, species migration, and evolution. In the short term, there are rapid changes in ecosystem conditions due to disturbances, such as storms and timber harvest. Furthermore, short-term disturbances also contribute to more gradual changes that ripple through the system. Over time, the ecosystem may change in subtle or catastrophic ways, with disturbances playing a key role. The biotic community may change due to changes in proportions among species and/or appearance of new species.

Although ecosystems are continuously changing over weeks, seasons, and decades, they tend to remain within the familiar range of conditions that have occurred repeatedly over centuries. This range of conditions for an ecosystem is called its "historic range of variability" (Morgan et al. 1994). To some extent, ecosystems are resilient to upheaval because they have evolved in association with upheaval. However they can be driven beyond the historic range of variability by natural events, such as volcanic eruptions, as well as human influences. Presently, due to the combined influence of natural events and modern human impacts, ecosystems are more likely to go outside their normal bounds. To evaluate present conditions in relation to the historic range of variability, we need to discover relevant ecosystem attributes that can be monitored over time to reveal trends and provide insight (Table 4).


Table 4. Potential indicators of stream ecosystem integrity.


Trends in riparian vegetation


Surveys of large woody debris in streams -- gives indications about habitat diversity and fish habitat quality.


Sampling of benthic macroinvertebrates (stream insects) -- gives indications about stream habitat characteristics including water chemistry, water temperature, sedimentation, flow regime, and food availability.


Estimate rates of bedload transport by measuring rate of fine sediment deposition in pools. Use data to estimate rates and trends in bedload transport and storage -- gives indications about stream response to disturbance.


Estimate fine sediment supply through pebble counting procedures -- gives indications about stream response to disturbance.


Pool - riffle - run ratio -- gives indications about habitat complexity of the stream.


Width-to-depth ratio -- gives indications about relative aggradation of the stream.


Stream channel stability


Stream channel sinuosity


Lag time between rainfall and runoff


Disturbance / recovery status and trends - take a snapshot" of the current "disturbance-recovery status" in watersheds and subwatersheds. Use criteria such as peak flow data, condition of riparian vegetation, land cover characteristics, and number of landslides. Eventually analyze a series of snapshots to evaluate "rate of recovery" in watersheds and subwatersheds.


Historic cycles of disturbance-recovery – gather available historic information about disturbances and recovery and compare to current conditions, trends, and cycles.


Another descriptor of ecosystem dynamics is "recurrence interval" (or "return period") for a particular ecosystem condition or event. Any ecosystem event or condition within the historic range of variability is expected to recur eventually. Recurrence interval is the average time between repetitions of a particular ecosystem event or condition (Botkin 1994). The normal range of conditions in an ecosystem and their pattern of occurrence provide a standard to evaluate present conditions, trends, and management options. The average recurrence interval should not imply that ecosystems are predictable. While some events and conditions occur in an irregular pulsing pattern, other events are entirely sporadic. For example, in California, precipitation occurs in a somewhat predictable pattern. However, droughts and record-breaking storms occur sporadically and unpredictably.

Another tool for assessing ecosystem conditions and trends is "indicator species". An indicator species is a sensitive species that is monitored to detect trends in habitat quality. The status of indicator species is used to infer the status of many other species that depend on similar habitat (Meffe and Carroll 1994). Like the canaries that were used by miners to warn of the presence of toxic gases, indicator species provide an early warning of ecosystem decline.

In stream ecosystems, benthic macroinvertebrates and anadromous salmonids are important as indicator species. Benthic macroinvertebrates are aquatic insects that are sensitive to stream habitat conditions. Through sampling benthic macroinvertebrates from gravel in the stream bed, the recent history of water quality can be determined (Plafkin et al. 1989). Anadromous salmonids are an indicator species on a very broad spatial scale because their life cycle requires favorable conditions in many habitats at different times. Their abundance is controlled by conditions in all the ecosystems they use (stream, river, estuary, ocean) as well as in migration corridors between these habitats.

Additional insights are possible through studying the controlling influences for river ecosystems, including high stream flows, geomorphic processes and riparian vegetation. Driven by the combination of stream flow and geomorphic processes, stream habitats and nearby riparian vegetation vary in a pulsing cycle of destruction and recovery. Peak stream flows and associated landsliding reduce the quality of stream habitat for fish. This is followed by a recovery period and a corresponding improvement in fish habitat. Although disturbances degrade fish habitat, they are nevertheless necessary to maintain high quality fish habitat. This is evident in river reaches below dams where floods do not occur and native fish populations decline. Between excessive disturbance and lack of disturbance, there is a range of recurrence intervals for stream disturbances that provide favorable conditions for native fish. It is a paradox that ecosystem integrity and fish habitat in coastal streams of the Pacific Northwest are maintained through alternating periods of upheaval and stability. The entire river ecosystem is adapted to and depends on an irregular pattern of upheaval and recovery. Over years and decades as disturbance and recovery vary across the watershed, the location of high quality habitat changes and population densities of species shift accordingly.

During the upheaval and recovery of stream habitats, there are important interactions between high stream flow and riparian vegetation. High flows periodically rip out riparian trees and prepare seed beds for the next generation of trees. The age class structure of riparian species, includnig alders, willows, and cottonwoods, is controlled by peak flows, bedload transport, and other related geomorphic processes. Riparian vegetation tells the story of high water events and associated changes in the stream channel over decades.

Another concept relevant to ecosystem integrity is "keystone species". For example, in many river systems, riparian trees have far-reaching influences on stream habitat, aquatic species composition, and geomorphic processes. Riparian tree species can be considered “keystone species” which means they play an exceptionally strong role in community processes or structure. Decline of a keystone species results in decline of related species and drastic changes in community composition and the food chain (Meffe and Carroll 1994). Removal of riparian trees typically results in less shade, higher water temperatures, and decline in native fish species. Furthermore when riparian trees fall in the stream, they are an important catalyst in forming critical habitat for anadromous salmonid fish stocks such as spring Chinook and summer steelhead. In turn, anadromous salmonids may also be a keystone species by virtue of their role in nutrient cycling. Over long periods, anadromous salmonids may be necessary for cycling phosphorous from the marine environment to inland areas (Cooperrider and Garrett 1995).

Evaluating ecosystem integrity is complex and difficult. It requires an estimate of the total system response to many known and unknown factors. In the case of river systems that support anadromous fisheries, we need to envision and evaluate conditions throughout large river basins over a long time. This large scope of research is expensive and slow. Furthermore, there are logistical problems due to administrative boundaries, such as state lines, and short time frames imposed by political election cycles. Due to these limitations as well as the limited capacity of humans to understand complex systems, there is scientific uncertainty about the effectiveness of any strategies. Simultaneously, there is disagreement about the degree of protection to give ecosystems in the face of economic demands. Despite this confusion, there is immense pressure for immediate action.


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